Composites of fiber-reinforced rubber are useful for many purposes including pneumatic tires and industrial belts. Typically, such reinforcing fibers are in the form of a cord comprised of cabled and twisted yarns. Such fibers can be of various materials including nylons, polyester, wire, rayon, cotton and glass.
An important requirement of such reinforced rubber composites is the adhesion of the reinforcing fiber or cord to the rubber itself. Such adhesion is of particular importance where the resulting composite is subjected to considerable flexing and distortion, such as the contortions encountered in the typical operation and use of pneumatic tires and industrial belts.
Various methods have been proposed and used to obtain adhesion of fiber or cord to rubber. For example, the cord has been pretreated prior to adhering to rubber by dipping, followed by drying, in an aqueous emulsion comprised of a resorcinol/formaldehyde latex (RFL), a latex treatment well-known to those having skill in the art, in which the resorcinol/formaldehyde resin is a part of an aqueous latex, usually a butadiene/styrene/vinylpyridine terpolymer. The resorcinol/formaldehyde resin is understood to be a primary element of the adhesive between the cord and the rubber with the latex being useful in reducing the modulus of the resin. Other advantages and features of the RFL for adhering cord to rubber are generally well-known to those having skill in the art.
While numerous methods are used to improve the adhesion of nylon, wire, rayon, cotton and glass to rubber, polyester fiber reinforcement has proven to be especially difficult. Polyester tire yarn requires the application of an adhesive layer to obtain bonding to the rubber. Two types of adhesive systems, a single dip and a double dip adhesive system, have been developed to meet this need.
In the double dip system, polyester yarn is treated with a first dip which is a dispersion of a phenol-block methylene bis phenylene diisocyanate, an epoxy resin, wetting agents and water. The treated cord is cured, then treated with a second dip of resorcinol-formaldehyde-latex and cured again.
In the single dip system, the adhesive layer is applied to the polyester yarn subsequent to drawing of the yarn. The polyester yarn is piled into cords which are treated with a resorcinol-formaldehyde-latex dip and cured. The need for the diisocyanate dip in cord processing is eliminated by the system. Low carboxyl polyester yarn treated with this system has very good ammonolytic and hydrolytic stability, but has poor adhesion to rubber when made into tire cord.
U.S. Pat. No. 4,356,219 relates to a method for preparing drawn, coated polyester yarn which comprises applying an aqueous emulsion of an epoxy resin and spin finish fiber treatment oil to spun or drawn yarn. The oil is comprised of a mixture of pentaerythritol tetraester fatty acid, polyoxyethylenenonyl phenol and oleyl diethyl imidazoleimium sulfate.
U.S. Pat. No. 4,300,615 claims the use of an adhesive dip for reinforcing fabrics which contains an antidegradant.
U.S. Pat. No. 4,472,463 discloses a second step dip for coating a cord or fabric made of polyester, wherein the dip is an aqueous emulsion containing an adhesive latex of a diene polymer and an acrylic resin.
European Patent Application Publication Number 0119948 discloses a polyester yarn having a coating thereon comprised of (A) a specified fiber treatment oil and (B) an epoxy resin containing a specified dioxaborinane.
U.S. Pat. No. 3,749,758 relates to compositions comprising carboxyl containing polyesters (prepared by the reaction of a dicarboxylic acid and a 2,2-di-(hydroxymethyl)carboxylic acid), and di- and/or polyepoxides.
U.S. Pat. No. 3,301,804 teaches that tire yarns treated with an aqueous emulsion of an epoxy novalac and an amino boron catalyst have excellent adhesion to rubber. The adhesion is satisfactory in a static composite, however, in dynamic applications such as in a tire, the stiff and brittle epoxy novalac experiences a dramatic loss of adhesion.
The prior art does not suggest or disclose that significant increases in adhesion of polyester fiber to rubber can be achieved through treating the polyester fiber with an aqueous emulsion of an isocyanate terminated polyester and an epoxy resin.